Apparatus for the homogenization and mixing of nuclear fuels

ABSTRACT

An apparatus for mixing and homogenizing U-235 or plutonium containing nuclear fuels is provided. It comprises a vertical, flat rectangular container which is either rotatable about a horizontal middle axis or is a fluidized bed. The apparatus has filter candles to separate exit gases from solids.

United States Patent 1191 Pirk et al.

1451 July 17,1973

[ APPARATUS FOR THE HOMOGENIZATION 1 AND MIXING OF NUCLEAR FUELS [75] Inventors: Hans Pirk, Dornigheim; Fritz Pliiger, Kleinostheim; Horst Vietzke, Grossauheim, all of Germany [73 Assignee: Nukem Nuklear-Chemie und- Metallurgie, GmbH, Wolfgang near Hanau am Main, Germany [22] Filed: June 23, 1969 [21] Appl. No.: 835,508

'[52] U.S. Cl. 259/3, 23/230, 259/81 R [51] Int. Cl B0lf 15/02 [58] Field of Search 259/3, 14, 30, 49,

259/50, 81 R, 81 A; 23/230 R, 286; 210/510 [56] I References Cited UNITED STATES PATENTS 2,689,178 9 1954 l-lignett 210 510 X 3,389,070 6/1968 Berghaus 259/3 X FOREIGN PATENTS OR APPLICATIONS 728,538 4/1955 Great Britain 259 81 R Primary Examiner-Robert L. Wolfe Att0rneyCushman, Darby & Cushman [5 7 1 ABSTRACT An apparatus for mixing and homogenizing U-235 or plutonium containing nuclear fuels is provided. It comprises a vertical, flat rectangular container which is either rotatable about a horizontal middle axis or is a fluidized bed. The apparatus has filter candles to separate exit gases from solids.

4 Claims, 4 Drawing Figures APPARATUS FOR THE l-IOMOGENIZATION AND MIXING OF NUCLEAR FUELS The processes for the production of nuclear fuels generally contain as the final step although sometimes also as an intermediate process step, the preparation of homogeneous, large batches having exact specifications, so-called lots.

It is desired to prepare the largest lots possible, since specification control as a rule is drawn on the lot, independent of its size (weight). One therefore keeps the expenses small for the analysis of the physical and chemical properties of the lots, and if necessary the testing of the work-ability, for example, of powders to pellets. The lot size is limited, however, by other factors such as technical-economical mixer size, equipment throughput and residence time of valuable materials in the equipment. The optimum lot sizes to be calculated under these postulations are considerable and amount to about 2 metric tons, for example, for UO powder with U-235 content of 4 percent and for a Uo -PuO mixture in the proportions by weight of 70 30 about 60 kg.

The production of such lot sizes results in increased difficulties according to experience, especially with powdered nuclear fuel which contain the fissionable isotope U-235 in higher than the natural concentration or contain Pu. The guarantee of nuclear safety limits the handling of the so-called safe amounts of fissionable material in any geometric arrangement or the handling of any amounts in physically fixed geometric arrangement, the so-called safe geometry. For UO powder with a U-235 concentration of 4 percent, for example, the safe amount is only about 26 kg. of U This amount can, under fixed provisions still be exceeded, whereby the guarantee of nuclear safety depends on the following of administrative instructions.

By the use of the so-called geometric safety as safety parameters the safe volume, the safe diameter or the safe layer thickness can be selected. At 4 percent enriched U0 powder amount, for example, the safe volume limit is 33.8 liters, the safe diameter limit is 28.45 cm. and the safe layer thickness limit is 13.97 cm.

It is evident that with a safe volume no substantial lot size can be prepared.

Mixing and homogenization apparatus with safe diameters are known, such as vertical, thin cylindrical vessels, in which the mixing action takes place through batchwise or uniform whirling up of the material to be mixed. However, also with this apparatus the desired optimum lot size cannot be reached.

The above-described principle boundaries for the guarantee of nuclear safety can be expanded or even disregarded if the multiplication factor of the amount of nuclear fuel to be treated is held under the critical value through homogeneous or heterogeneous poisoning with neutron absorbers.

The homogeneous poisoning is practically not usable for the above case since a later separation of the neutron poison is possible only through an expensive chemical separation.

The heterogenous poisoning of the fissionable material is not applicable especially in treating mixing problems, since just the inserted absorber plates and rods strongly hinder a good intermixing, besides leading to expensive and difficult cleaning of the mixing apparatus.

A frequently used practice is to prepare larger lots through repeated crossing mixing of homogeneous smaller single lots. This method is very expensive, however, lacking better technical processes solutions it is tolerable.

It has now been found the pictured difficulties in the preparation of larger homogenous amounts of nuclear fuels also can be avoided in mixing containers of abnormal geometry if these are made according to the apparatus of the present invention with use of geometrically safe layer thickness. Thereby, it is possible for the first time to accomplish in an apparatuswise comparatively simple arrangement with small expenditure of work the homogenization and mixing of larger production units. The found apparatuswise conception yields above that further advantages in as much as the mixer on account of its apparatus-wise simple construction also can be used as a cheap stock container and furthermore can be quickly cleaned in a very simple manner, which is of substantial significance, especially in the customarily frequent product change in the production of nuclear fuel.

The use of the principle of safe layer thickness leads to two different, however, equally well suited, apparatus-wise simple mixers, a so-called free fall mixer and a so-called forced mixer.

The invention will be understood best in connection with the drawings wherein:

FIG. 1 is a schematic front elevation of the free fall mixer apparatus;

FIG. 2 is a schematic side elevation of the mixer of FIG. 1;

FIG. 3 is a schematic frontillustration of the force mixer apparatus; and I FIG. 4 is a side elevation of the mixer of FIG. 3.

The free fall mixer is essentially a vertical mixing container of safe layer thickness slowly rotating about a horizontal middle axis.

As shown in FIGS. 1 and 2, the mixer 9 with nuclear safe layer thickness is mounted for rotation in the direction indicated about its central axis 7. The mixer has a powder entrance for powder and conveying air, a mixture exit 2 including a valve 8, blow back filter candles 3, purified waste gas exit conduit 4, pressure gauge box 5, motor 6 A.

The container 9 which is strongly reinforced on the outside is filled to a maximum of two-thirds its capacity with the amount of product to be mixed and homogenized, preferably powdered material. The locked container, is filled through sucking in the powders via a simple separator with more than 99 percent separating efficiency whereby the non-objectionable and purified conveying air is sucked off through installed blow back filter candles 3 made of sintercd metal. The filled mixing container is tightly closed off from the convcyer system and the container contents then thoroughly mixed for several minutes, maximum 15 minutes, at variable rotational speed of the container.

If one wishes to avoid expensive, permanent installations, for example, carrier shovels, which also make difficult purification of the container the rectangular cross sectional design of the mixer, particularly the square design, is approved. The continuous, adjustable drive of the mixer takes place through a pressurized air or hydraulic motor; the container contents are weighed with pressure measure capsules. The peripheral velocity of the mixing container is so regulated that the powder contents are partially taken up to shortly before the culmination point in order then to be transferred in an avalanche like flow, sliding, passing over itself motion toward the bottom, whereby the powder mass in the lower part of the mixer is deeply penetrated and thus a quick and thorough mixing is effected.

The thoroughly mixed or homogenized contents are sucked out of the container over a closable outlet having a plug of conical form whereby the conveyer air can be injected through filter plates around the exit openmg.

The forced mixer 20 as shown in FIGS. 3 and 4, however, without built-in mechanically moving parts is essentially vertical standing rectangular mixing container of safe layer thickness A with horizontal or slightly sloped bottom plate 21 made of porous sintered metal and divided into chambers 13 and in the container cover are found blow back filter candles 14 of sintered metal as well as the valve 1 1 and conduit 22 for loading and a dip tube for dischargingthe container. There is also provided a powder exit conduit 12 provided with valve 23. Purified air from the filter candles passes through valve 24 and conduit 15. The powder is maintained in the form of a fluidized bed 16 by means of air being blown through the separate chambers of the gas distribution plate.

The filling of the preferably powdered substance occurs similar to that in the described free fall mixer. The thorough mixing of the powder takes place by fluidizing with air which as stated is blown in through the chambers of the porous bottom plate in such a way that the vertical toward the top directed gas and powder transport trend is overlapped by a horizontal and even toward the bottom directed transport component. This is attained in a very simple way through a variable, automatic, preselected air throughput through the separated air distribution chambers. The regulator is indicated at 29. The necessary mixing times are less than 1 hour.

The transporting out of the mixed or homogenized powder is accomplished through the vertical tube dipped from above with central feed of conveyer air or in the manner described in connection with the free fall mixed.

Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1 There was taken 200 kg. of homogeneous U0,

powder and as described introduced in a forced mixer with the dimensions, height 1,300 mm., width L000 mm. thickness 120 mm. To this powder was further added 6 kg. of fuchsin colored U0, powder from the same source.

The powder was then fluidized and thoroughly mixed for 40 minutes with a specific amount of air of 95 Nmlm per hour wherein at 2 minute intervals powder samples were taken from the central bottom discharge. From these samples the dye was extracted and the extinction of the colored solution arising measured. This method is well suited by experience for determination of mixed materials. In this test a sufficient homogeneity of the mixing was reached after 30 minutes.

EXAMPLE 2 In the forced mixer of safe thickness corresponding to example 1, there were introduced and fluidized I35 kg. of U0, powder of normal quality. In the fluidized bed?" there were also added 7.5' kg. of U 0 powder manufactured by oxidation of sintered bodies and 7.5 kg. of U0, powder produced from ground mud by drying and passing through a sieve, fluidized and thoroughly mixed for 30 minutes with a specific amount of air of Nm lm per hour.

The homogeneity of the charged powder was confirmed though unobjectionableness in several determinations of specific powder poroperties and in a subsequent factorial sinter test. The sinter results obtained lie within the usual narrow variation breadths.

EXAMPLE 3 EXAMPLE 4 ha free fall mixer having the following measurements; height 3,500 mm., width 3,500 mm. and thickness mm. (safe layer thickness up to 4 percent U 235) there were introduced 13 production charge of U0, powder of different powder quality, all together 2 tons of U0, with a U-235 content of 4 percent over a rotating separator.

The 2 ton powder charge was homogenized by turning the geometrically safe mixing container with 2 revolutions per minute (0.5 m/sec. maximum peripheral velocity) in the course of 30 minutes.

After ending the mixing the U0, powder was pneumatically discharged from the stopped mixer over a discharge apparatus. From the powder stream flowing off powder samples were taken in equal qunatitative intervals and investigated as to their properties. The found values for the separate powder properties lay completely inside the accuracy of determination.

We claim:

1. Apparatus for mixing and homogenizing fissile nuclear fuels in liquid or solid phase comprising: a flat container disposed with its longer dimension in a vertical plane, said container defining an interior rectangular space of nuclear safe thickness; means mounting said container for rotation about a generally horizontal axis which passes through the container in the thickness direction; scalable inlet means for introducing gas suspended fissile powder into said container; scalable waste gas outlet means for removing the suspension gas from said container, said outlet means including filter means within said container for preventing removal or fissile powder with the suspension gas; and separate sealable outlet means for removing fissile powder from said container.

2. Apparatus as in claim 1 wherein said container is square as viewed along the axis of rotation.

3. Apparatus as in claim 1 wherein said filter means includes a porous sintered metal filter disposed within said container and having a face in communication with a conduit which forms part of said waste gas outlet means.

4. Apparatus as in claim 1 including means for rotating said container about said axis. 1 i t t I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 746 312 Dated July 17 I 9'71 Inventor(s) Hans Pirk et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading, line 30 was omitted and should appear as follows:

[30] Foreign Application Priority Data April 12, 1969 Germany P l9 18 675.5

Signed and sealed this 16th day of July 1974;

(SEAL) Attest:

MCCOY M. GIBSON, JR. 0. MARSHALL CDANN Commissioner of Patents Attesting Officer USCOMM-DC 6O376-P69: u s. (.ovumupu mumm- OHKI nn 0- aye-1J4 'ORM P0-1050 (10-69) 

2. Apparatus as in claim 1 wherein said container is square as viewed along the axis of rotation.
 3. Apparatus as in claim 1 wherein said filter means includes a porous sintered metal filter disposed within said container and having a face in communication with a conduit which forms part of said waste gas outlet means.
 4. Apparatus as in claim 1 including means for rotating said container about said axis. 